1. Field
The present patent specification relates to an optical recording medium having improved optical properties, and a method for fabricating the same.
2. Description of the Related Art
Recently, write-once compact disks (CD) are widely available in the market. A write-once CD differs from conventional CDs in that the user can record information on it, and the signals recorded on the write-once CD also satisfy the specifications of conventional CDs and therefore can be reproduced using a commercially available CD player. One method for fabricating such a recording medium is described in Japanese Patent Application Laid Open No. Hei 2-42652, which include forming a light absorbing layer by spin-coating a substrate with a dye and then forming a metallic reflecting layer on the back side of the substrate. Moreover, Japanese Patent Application Laid Open No. Hei 2-132656 discloses a currently commercialized write-once CD, in which the complex refractive index and the thickness of the light absorbing layer are suitably adjusted to make the recorded signals satisfy the CD specifications. However, the dye-containing write-once CDs disclosed in Japanese Patent Application Laid Open No. Hei 2-42652 and No. Hei 2-132656 both do not have sufficient light resistance. That is, the signal characteristics of the CDs will change after exposure to sunlight for a long time or the like, and therefore will not satisfy the CD specifications thereafter. The drawback is caused by light-induced deterioration of the dyes, especially the cyanine dyes, used in the light absorbing layer.
A method for inhibiting the light-induced deterioration of dyes is to add a light stabilizing agent in the light absorbing layer, as described in Japanese Patent Application Laid Open No. Sho 63-159090. However, as the addition amount of the light stabilizing agent is small (<20%), the light resistance is insufficient. The addition of large amount of light stabilizing agent (>20%) will alter the optical and/or the thermal properties of the light absorbing layer, and the quality of signals is lowered in every aspect. On the other hand, Japanese Patent Application Laid Open No. Hei 3-62878 discloses a write-once CD that uses phthalocyanine compounds, which are dyes having a high degree of light resistance. In such a write-once CD, the required complex refractive index of the light absorbing layer, which is formed by spin-coating a substrate with phthalocyanine compounds, is obtained by incorporating specific functional groups to the phthalocyanine compounds. Thereby, a write-once CD having high light resistance can be fabricated. However, as compared with the conventional write-once CDs that use cyanine dyes in the light absorbing layer, the one disclosed in Japanese Patent Application Laid Open No. Hei 3-62878 is of inferior quality in the aspect of reproduced signals. That is, the length (pit length) of each signal in the reproduced signals tends to differ from the theoretical pit length of the original CD signal, and therefore reproduction errors easily occur.
One method for solving the aforementioned problem is described in Japanese Patent Application Laid Open No. Hei 11-48613, which is a prior application of the Inventors. In the method, phthalocyanine compounds that have thermal decomposition temperatures ranging from 350° C. to 450° C., are mixed in specific ratios, so as to inhibit the variation of pit lengths. However, as the recording linear speed continuously gets higher, the noise jitter of the write-once CD disclosed in Japanese Patent Application Laid Open No. Hei 11-48613 is significantly increased, and the characteristics of the write-once CD are deteriorated. Moreover, Japanese Patent Application Laid Open No. Hei 10-181204, which is also a prior application of the Inventors, discloses an optical recording medium that includes a light absorbing layer mainly composed of two phthalocyanine compounds having a thermal decomposition temperature from 250° C. to 350° C. and a thermal decomposition temperature from 350° C. to 450° C., respectively. The optical recording medium has better signal characteristics and higher light resistance and causes less reproduction errors, but does not have suitable recording characteristics over a wide range of recording linear speed.